You wanna put a motte around your castle, CONSIDER building a false-wall assembly to recess int the table. Then seal it (silicone, whatever; You may have to experiment a bit to figure out what will work best.)
EDIT: Here’s a diagram of how it lays out; Please keep in mind that this particular diagram wasn’t actually meant to address this sort of design so it’s a bit on the “rough” side.
Red indicates dirt, wood, or whatever you set/mount the castle itself to (you could actually build a table/false deck of PM itself), light blue indicates water (or gel, or whatever you fill your motte with).